Ok little boy! You will learn few things as you grow up! First school isn't about truth! Experience IS!

Ask a microbiologist what he thinks of viruses! He will tell you that their origin isn't likely to come from our own planet as they defy what we describe living entity! Recently they found that virus do also have even smaller virus attacking them! Meaning that a virus can infect another one!
But viruses are even weirder than this! They can form in space and survive irradiations!
Also some of them might even been formed inside the radiation cloud of super nova, where BE-10 radiations help their formation. But this you won't be capable to do such links until you dig deeper with your own questionings!

Peer review papers are necessary for scientists when it comes to new discoveries... I like to be ironic... Sorry if you do not like ; )

As for the geology of our planet, it is important to know that life has shaped its mineral composition to very high level! Increasing the conditions needed for life to thrive!, Acidic levels and alkaline levels in the ground have been stabilized, filtering our atmosphere, making it proper to sustain denser patches of life! Stones and minerals do act as filters for water (like rivers or lakes) and our atmosphere! And this you should know this! So if intelligent life has to appear it will be due to the long work of life activities on the surface and beneath the surface of their planet! Meaning that life can even change a planet in ways that make it more viable toward evolution of complex biological entities, even if at first the conditions weren't optimal!

As for your opinion that Symbiogenesis makes it even less likely for life to appear is mainly out of surface reasonings! If you learn a bit of advance biochemistry, you learn that what puts all our proteins together is called Chirality (racemic molecules or non-racemic). This Chirality order is the same for all life on this planet, and only molecules created in laboratory do not have the same! Actually, science do have a really hard time at figuring out the primordial soup that have seen life arising from! There are many recipes but one of the most important variable that have been overlooked is the action of the Earth magnetic field on biochemistry! It has shaped our molecules Chirality and determined their level of interactions! (I can provide a "peer review paper" if you like!!)
So out of this mess, when you close up a bit into the details of how life did appear, you can feel a certain order of things! And order that have guided the development of ALL life, in and out of the ecosystem! Things makes senses but sciences do have a hard time to prove everything that works in nature and even more so outside our little blue planet!
So yes, you can sit and ponder that science hasn't YET proved that Virus do indeed come from outerspace, to make you feel comfortable about it, but the truth is that science isn't even close to prove anything such as this, so do you think that we should wait and sit because of this?
Schools teach you 2 things: how to structure your mind to fit inside the system called society, and show you how determining the interactions with peoples will be on your professional life (what you do will mostly depend of who you know if you reach a certain level).
But it doesn't have the arrogance of saying, HEY school IS the truth!! YOU will experience it because you will have learned how to learn from LIFE itself! It is why the world is lead by peoples over their 40's, because they have experienced life, not only learn about it!
As for me, I do not care much about scientific reviews, or peer reviews, as it has been a while that I have proved myself that my hypothesis were correct and it has been a while that I understood that most peoples in my field were full of arrogance and seeing only what they want to see, with so little imagination that they couldn't even see how their mother loved them!

So, yes in my opinion and from a scientific point of view intelligent life is very possible outside our little planet, but ONLY if you get out of the box and begin to think for yourself instead of being induced what YOU MUST think!
And you must also know that saying such thing as Virus coming from space is crazy, also undermine your credibility when you know that virus origin is unclear and that science didn't have found the answer to this yet!

So, knowing that virus do cause mutation in our DNA, and that our Human brain have mutate 2 times in the last 40000 years, we might surmise that those mutations have been triggered by some kinds of virus since its about the only horizontal gene transfer mechanism out there. And you must admit that viral DNA composing 43% of our genome is incredibly high for a complex life form like us! So these horizontal gene transfers have help shape humans as we are now! And on a evolutionary scale, our bipedal model is the most economic (considering how much energy our brain do consume), and the most likely form for another advance life form!

Anyhow, yes there are a lot of conjectures! You might believe what is the most comfortable for you at this point, it won't matter! But with experience you will discerns whats making senses, and whats not! So you were asking why intelligent was possible outside our planet, so I answer with my opinion, as no scientist can really answer this with absolute evidences! But life is much more resilient than you think, and when you come to know her, up and close you understand that there is MUCH we can't even understand yet! Life is much more complex than she seems, and the process of life in this Universe is much more powerful than you might think!
But this only time will teach you!
On this, have a great time finding your answers!